Currently, 3rd generation cellular communication systems are being rolled out to further enhance the communication services provided to mobile users. The most widely adopted 3rd generation communication systems are based on Code Division Multiple Access (CDMA) and Frequency Division Duplex (FDD) or Time Division Duplex (TDD) technology. In CDMA systems, user separation is obtained by allocating different spreading and/or scrambling codes to different users on the same carrier frequency and in the same time intervals. In time division multiple access (TDMA) systems user separation is achieved by assigning different time slots to different users.
In addition to TDMA, TDD provides for the same carrier frequency to be used for both uplink and downlink transmissions. The carrier is subdivided in the time domain into a series of timeslots. The single carrier is assigned to uplink during some timeslots and to downlink during other timeslots. An example of a communication system using this principle is the Universal Mobile Telecommunication System (UMTS). Further description of CDMA, and specifically of the Wideband CDMA (WCDMA) mode of UMTS, can be found in ‘WCDMA for UMTS’, Harri Holma (editor), Antti Toskala (Editor), Wiley & Sons, 2001, ISBN 0471486876.
In order to provide enhanced communication services, the 3rd generation cellular communication systems are designed to support a variety of different services, including packet based data communication. Likewise, existing 2nd generation cellular communication systems, such as the Global System for Mobile communications (GSM) have been enhanced to support an increasing number of different services. One such enhancement is the General Packet Radio Service (GPRS), which is a system developed for enabling packet data based communication in a GSM communication system. Packet data communication is particularly suited for data services which have a dynamically varying communication requirement such as, for example, Internet access services.
Low chip rate TDD (LCR-TDD) is a TDD system that is part of the third generation set of technologies. In contrast to other third generation technologies, LCR-TDD employs a chip rate of 1.28 Mcps. The LCR-TDD technology also has a unique frame structure 100, as illustrated in FIG. 1.
Here, a 10 msec frame 100 consists of two 5 msec sub-frames. Contained within the 5 msec sub-frame boundaries 105, a first time timeslot 110 is typically dedicated for a downlink beacon transmission of duration 75 μsec. A downlink pilot time slot (DownPTS) field 115 is then sent for downlink synchronisation of duration 75 μsec. An uplink pilot time slot (UpPTS) 125 of duration 125 μsec is similarly used for uplink synchronisation.
A guard period (GP) 120 of duration 75 μsec is located between the uplink and downlink pilot time slots 115, 125. Following the uplink pilot time slot (UpPTS) 125, a number of Uplink traffic slots 130 and downlink traffic slots 140 are transmitted, each of duration 675 μsec. Thus, within an LCR-TDD timeslot, multiple channels (or multiple users) may be multiplexed together using code division multiple access (CDMA). An UL/DL switching point 135 defines the changeover in operation of the LCR-TDD sub-frame from UL transmissions to DL transmissions.
It is noteworthy that there is some variability in the LCR-TDD sub-frame in that the position in time of the UL/DL switching point 135 can be moved within the radio sub-frame. FIG. 1 shows the case where the number of uplink traffic slots 130 is equal to the number of downlink traffic slots 140. This, however, may be varied between the case in which there is only one downlink traffic slot per sub-frame and the case in which there is only one uplink traffic slot per sub-frame (at least one slot in each link direction must be present to facilitate bi-directional communication).
Recently, significant effort has been invested in designing a new air-interface, termed E-UTRA, for use within 3GPP systems. The new E-UTRA air-interface can be operated in unpaired spectrum using a TDD mode or in paired spectrum using a FDD mode; it is based on orthogonal frequency division multiple access (OFDMA) in the downlink channel and either OFDMA or frequency division multiple access (FDMA) in the uplink channel. The E-UTRA air interface is time-slotted and multiple channels (or multiple users) can be multiplexed together through frequency domain multiple access techniques (OFDMA and FDMA). The timeslot duration of E-UTRA is 0.5 msec.
It is envisaged that E-UTRA can be deployed with a low latency structure where it is possible to switch between downlink timeslots and uplink timeslots on a per timeslot basis, i.e., allowing the link to be rapidly switched. This enables low latency transmission and re-transmission by virtue of the ability, thus offered, for the receiving end to quickly acknowledge receipt of a data packet by transmitting an acknowledgement in the reverse link direction. An alternative higher-latency structure is also possible where switching between downlink timeslots and uplink timeslots would occur on, say, a five timeslot basis.
Within the 3GPP standard's committees, where the communication system's specifications are being defined, there has been a significant amount of discussion on the ability of different technologies to co-exist within the same or adjacent frequency bands.
It is known that a subscriber unit, referred to as user equipment (UE) within 3GPP, or a wireless communication serving unit, referred to as Node-B within 3GPP, that transmits at one carrier frequency inevitably ends up also transmitting some energy at frequencies that are outside the nominal bandwidth of the carrier frequency. Hence, energy is emitted not only in the intended (frequency) band, but also in adjacent bands, as illustrated in FIG. 2.
Referring now to FIG. 2, a spectral plot 200 of receiver characteristics 215 and transmitter characteristics 220 is shown, with frequency 210 plotted against power spectral density 205. Thus, FIG. 2 shows transmissions 220 at a carrier frequency f1, which comprise leakage energy 225 that interfere with reception in adjacent bands f0 and f2.
In a cellular deployment, multiple frequencies may be used by the same operator. In addition, a single Node B may be configured to control multiple frequencies. Alternatively, the Node-B may be configured to control a single frequency. The Node-Bs that serve these different frequencies may be either co-located or may be located in different cell sites. Users may be located anywhere in the geographic area of the Node-Bs, i.e., users may roam or move around a particular geographical area supported by a single Node-B or by multiple Node-Bs.
This interference problem is explained further in FIG. 3, which illustrates a situation 300 where there is UE to UE interference in an unsynchronised TDD system. FIG. 3 illustrates a first Node-B-1 305 that transmits to a first UE (labelled “UE-1”) 315 on a downlink carrier frequency f0 (for example, the carrier frequency labelled f0 in FIG. 2) 310. FIG. 3 also illustrates that, at the same instant in time, a second UE (labelled “UE-2”) 320 transmits to a Node-B (labelled “Node-B-2”) 330 on an uplink carrier frequency f1, (for example, the carrier frequency labelled f1 in FIG. 2) 325.
In FIG. 3, it is assumed that UE-1 315 and UE-2 320 are located a significant distance from the Node-Bs 305, 330, that they are communicating with. Hence, UE-1 315 will receive the transmission from Node-B ‘1’ at a low level, where the first Node-B 305 may only be able to maintain a link to UE-1 315 by using a low coding rate, allowing the first UE-1 315 to receive at a low power level. UE-2 320 will transmit to Node-B ‘2’ 330 using a high power, in an attempt to maintain a communication link. If UE-1 315 and UE-2 320 are closely located, then the path loss between UEs 315, 320, will be minimal. In this case, spurious emissions from the uplink transmission of UE-2 320 will leak into the adjacent frequency f0 and will significantly impair reception of the transmission to UE-1 315.
A variety of solutions have been identified, in the field of wireless cellular communications, to address the problem of operating a new TDD technology with an evolved TDD air interface within the same geographic area.
A first solution, which has been proposed within the 3GPP standards forum, is to use a frequency guard band between a carrier used for E-UTRA and a carrier used for UTRA TDD. Use of a guard band works on the principle that although leakage of energy into an adjacent carrier might be significant, leakage of energy into carriers that are further separated in frequency is less significant. Thus, the guard band approach sacrifices those carries in which there may be significant leaked energy, i.e., these carriers are not used. This approach is wasteful of spectral resource, but is a simple solution to the problem.
A second solution is to specify amplifier characteristics and filter characteristics (such as through adjacent channel leakage ratio and adjacent channel selectivity specifications) in UEs and Node-Bs, such that the energy that they leak into adjacent channels is insignificant. This approach is not wasteful of spectral resource, but does increase the cost of UE and Node-B equipment.
A third solution would be to design E-UTRA to have an identical frame structure to the UTRA TDD structure, such that they operate synchronously. This approach is illustrated in the timing diagram 400 of FIG. 4. The approach is not wasteful of spectral resource, but does limit the performance and flexibility of E-UTRA. For example, with such a solution, the frame structure of E-UTRA must contain the same uplink/downlink switching points as UTRA TDD. Maintaining the same switching points as UTRA TDD will lead to the latency experienced by E-UTRA transmissions being of the same order as those of UTRA TDD transmissions, albeit that latency can be reduced in 3.84 Mcps TDD (HCR-TDD) by shortening the time between uplink and downlink transmissions, i.e., by increasing the number of switching points for both systems (noting that use of multiple switching points is not currently possible for LCR-TDD).
FIG. 4 illustrates a typical LCR-TDD frame structure 100, as shown in FIG. 1. In this example, a 3:3 DL:UL timeslot split for traffic data is shown. Also shown is a downlink timeslot that is typically used for beacon type transmissions in the first timeslot of the sub-frame and the DwPTS/GP/UpPTS fields, as previously mentioned.
FIG. 4 also illustrates a modified version of E-UTRA operating in an identical frame structure to LCR-TDD. In this mode, the E-UTRA sub-frame (also referred to as timeslot within a 3GPP context) duration is extended from 0.5 msec to 0.675 msec. In this mode, special sub-slots 415, 425 are inserted in the frame in order to facilitate coexistence between the LCR-TDD frame and the E-UTRA frame. These special sub-slots may either be idle (no data transmitted) or the UL special sub-slot may be used to transmit some uplink data, signaling or pilot information and the DL special sub-slot may be used to transmit some downlink data, signaling or pilot information.
Note that the frame structure illustrated in FIG. 4 has the at least the following disadvantages. For example, the E-UTRA frame is constrained to have two DL to UL (and two UL to DL) switching points per frame. This significantly impacts the minimum latency that can be achieved with such a frame structure. Furthermore, the E-UTRA sub-frame duration of 0.675 msec., when used in this compatibility mode, is different to the sub-frame duration of 0.5 msec that is used in paired spectrum.
It is noteworthy that LCR-TDD only operates in unpaired spectrum. This different sub-frame duration may also impact the design of the signal within the sub-frame. When E-UTRA supports two different sub-frame durations (as per the prior art discussed here), the design of UEs and Node-Bs that can operate in both paired spectrum and unpaired spectrum becomes much more complicated. This complexity increase will typically lead to a cost increase for UE and Node B equipment.
Thus, current techniques are suboptimal. Hence, an improved mechanism to address the problem of operating a new TDD technology with an evolved TDD air interface within the same geographic area would be advantageous. In particular, a system allowing for the provision of an E-UTRA TDD system to co-exist with a LCR-TDD system would be advantageous.